It's because designs have trade-offs and to accomplish the goal for drivers/frequency response/etc you have to make choices that impact the impedance. Those designing for consumer electronics may have a goal of 6-8ohms and those designing for hifi will probably have a goal range of 4-8ohms because their customers are likely to be able to handle the load and it gives them more flexibility in design trade-offs.

There are some hi-end speaker companies that build models that go even lower in impedance and require careful selection of amps.